1. Technical Field
The present invention relates to a sensor for sensing a parameter such as strain or temperature; and more particularly to a sensor system having a structured sensor fiber Bragg grating (i.e. multi-line spectral response) in combination with a structured xe2x80x98receiverxe2x80x99 or xe2x80x98readoutxe2x80x99 fiber Bragg grating with similar spectral characteristics. The spacing of the multi-line components of the structured sensor grating are different to that of the structured readout grating so that the two xe2x80x98beatxe2x80x99 to produce an amplified response to strain/temperature or other parameters as desired. The concept would apply to multiplexing using time-multiplexing or some form of frequency multiplexing. The xe2x80x9cmixingxe2x80x9d or xe2x80x9cbeatingxe2x80x9d may also be accomplished by electronically multiplying a structured sensor grating signal with a simulated reference grating signal to achieve the same result.
2. Description of Related Art
FIG. 1A shows a fiber grating based sensor, such as Bragg gratings, that typically exhibit a single, sharp resonance feature which is tracked as the fiber is subjected to strain or temperature variations (or other parameters via a suitable transduction mechanism). The wavelength shift with strain and temperature is normally expressed via the normalized response expressions:             (              1        λ            )        ⁢          (              δλ        δϵ            )        =                    ηϵ        ⁢                  
                (                  1          λ                )            ⁢              (                  δλ                      δ            ⁢                          xe2x80x83                        ⁢            T                          )              =          Δ      ⁢              xe2x80x83            ⁢      T      
For example, a fiber grating at a nominal wavelength of 1.3 micrometers, has a response of about 1 nanometer of wavelength shift per 1000 microstrain (0.1%) or a temperature of about 100 Celsius. This intrinsic responsivity is a function of the silica glass and waveguide parameters, and is thus a fixed quantity. (Slight variations could be induced by radically changing the host glass, doping the fiber, or by using certain waveguide geometries). The minimum change in strain (or other parameters) which can thus be detected and determined in large part by this responsivity factor. In the case cited above, if a grating wavelength shift of 1 picometer (i.e. 10xe2x88x9212 meters) can be detected, a 1 nanometer/1000 microstrain responsivity corresponds to a minimum detectable strain of about 1 microstrain. This responsivity, or xe2x80x98scale factorxe2x80x99, can be enhanced by utilizing a spectral beating effect.
FIG. 1B shows a grating with multi-spectral line components produced by taking a normal grating and overlaying an xe2x80x98amplitudexe2x80x99 modulation along its length. This modulation of the grating strength along its length produces xe2x80x98sidebandsxe2x80x99 in the spectral response of the device. These sidebands are spaced at an interval in wavelength space determined by the periodicity of the overlaid xe2x80x98super-structurexe2x80x99 grating amplitude modulation function. FIG. 1B shows the type of response typically observed.
The present invention provides a new and unique fiber Bragg grating sensor system for sensing a parameter, including temperature or strain, comprising: a structured sensor fiber Bragg grating in combination with a broadband light source, coupler, readout grating mixing and spectral analysis system.
The structured sensor fiber Bragg grating responds to an optical signal, and further responds to a sensed parameter, for providing a structured sensor fiber Bragg grating signal containing information about the sensed parameter.
The broadband light source, coupler, readout grating mixing and spectral analysis system provides the optical signal, and responds to the structured sensor fiber Bragg grating signal, mixes the structured fiber Bragg grating sensor signal with a reference grating spectrum, for providing a broadband source, coupler, readout grating mixing and spectral analysis system signal containing information about a mixed spectral analysis of the structured sensor fiber Bragg grating signal that is used to determine the sensed parameter.
In one embodiment, the structured sensor fiber Bragg grating signal is mixed with the readout reference grating spectrum by reflecting it off a structured readout fiber Bragg grating, for providing a combined structured sensor and readout fiber Bragg grating signal to a spectral analysis system.
In effect, the present invention provides a grating based sensor in which the wavelength response, or shift, to a particular parameter, such as strain, is amplified over that normally attained by using a combination of xe2x80x98sensorxe2x80x99 and xe2x80x98readoutxe2x80x99 gratings, each with multi-spectral line components. The spacing between these components is different for the sensor and readout gratings, and thus a beating between the two elements occurs which gives rise to an enhanced overall response.
If two of these structured gratings are used in combination, a beating effect between the two gratings is observed, which can result in an amplification of the effective scale factor of the system, and thus an improvement in the minimum detectable strain.
In another embodiment, the structured sensor fiber Bragg grating signal is analyzed with an optical spectrum analyzer, then electronically mixed with the readout reference grating spectrum by multiplying (weighting) it with a simulated reference grating spectrum signal in a spectrum weighting device. In this case, the xe2x80x9cmixingxe2x80x9d or xe2x80x9cbeatingxe2x80x9d process is accomplished electronically to yield the same result as in the first embodiment.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.